Carbon nanotubes (CNTs) have valuable qualities as structural materials. Potential uses can be found in fields as diverse as textiles, concrete, polyethylene, synthetic muscles, high tensile strength fibers, and fire protection. CNTs can be fabricated as electrical conductors, insulators, and semiconductors. A nanotube formed by joining two nanotubes of different diameters end to end can act as a diode, suggesting the possibility of constructing computer circuits entirely of nanotubes.